1. Field of the Invention
The present invention relates to a front light arranged in front of a display device to improve brightness and uniformity of brightness of the image plane on the display device, as well as to a reflective liquid crystal display device and a personal digital assistant.
2. Description of the Background Art
FIG. 57A is a schematic cross section representing an example of a conventional front light (Japanese Patent Laying-Open No. 8-94844). FIG. 57A shows a cross section of the front light as a whole, and FIG. 57B is a partially enlarged illustration of an upper surface of an optical guide plate. The optical guide plate 101 of the front light has a surface 101a (hereinafter referred to as the xe2x80x9cfirst surfacexe2x80x9d) opposing to the display device 130 and an upper surface 101b (hereinafter referred to as a xe2x80x9csecond surfacexe2x80x9d) opposite to the first surface, and on the second surface, a plurality of grooves each having a prism-shaped cross section are provided extending as ribs in the widthwise direction. The front light is placed in front of a reflective liquid crystal display device, that is, on the side of a viewer viewing the displayed image plane, and emits light toward the display device. In the front light 110, a light beam entering from a light source 103 to the optical guide plate 101 proceeds through the optical guide plate 101 away from the light source, while the beam is reflected by a reflective surface 121 of a steep slope, which is one surface of the prism on the second surface 101b, and emitted to the side of the first surface, approximately perpendicular to the optical guide plate. Therefore, as shown in FIG. 57B, the reflective surface forms an angle of about 43xc2x0 with a hypothetical bottom plane of the prism, and a gentle slope 122, which is the other surface of the prism, forms an angle not larger than 10xc2x0 with the bottom surface. The height of each prism is set to be 5 xcexcm to 50 xcexcm so as to reflect light of approximately the same intensity at various portions from end to end of the optical guide plate, to ensure uniform brightness in the plane. Along an end surface of the optical guide plate, light source 103 such as a cold cathode tube is arranged, and a light beam emitted from the light source enters the optical guide plate from that end surface and propagated through the optical guide plate. The light entering the optical guide plate propagates while repeating regular reflection at certain portions of the first and second surfaces 101a and 101b, and when the beam reaches the reflective surface 121 of the second surface, the beam is reflected and emitted approximately vertically, from the first surface 101a of the optical guide plate. Reflector plates 104 are arranged on an outer side of light source 103 and at the other end of the optical guide plate, so as to prevent any loss of light intensity. Further, the first and second surfaces are protected by a light transmitting layer 120. The light emitted from the optical guide plate enters the reflective display device 130, reflected by a reflecting member (not shown) arranged behind the liquid crystal, and returns to the optical guide plate 101. For convenience of description, in the following, the term xe2x80x9cdisplay devicexe2x80x9d refers to the xe2x80x9cdisplay apparatusxe2x80x9d with the front light excluded. During the progress, the light beam is modulated in the display device, and passes through the optical guide plate in the thickness direction to form an image on the screen, which is viewed by the viewer. When uniformity of light intensity in the plane is to be ensured by the front light, it is necessary to change area ratio of the reflective surface of the second surface 101b from portion to portion. It is impossible to change the area ratio of the reflective surface while keeping constant the angle of the prism structure. Therefore, in order to obtain high uniformity, it is necessary to change the angle of the prism structure. An optical guide plate having the prism angle varied in one same plane is difficult to process and to fabricate with high precision, resulting in higher cost.
FIG. 58A is a schematic cross section representing another example of the conventional front light (Japanese Patent Laying-Open No. 10-326515). FIG. 58A is a cross section of the front light as a whole, and FIG. 58B is a partially enlarged illustration of the second surface 101b of optical guide plate 101. The second surface 101b of the guide plate in the front light is stepwise, including a flat portion 123 that is parallel to the first surface, and a reflecting portion 121 forming an angle of about 45xc2x0 with the flat portion. The light beam proceeding through the optical guide plate encounters the reflecting portion 121 and reflected approximately vertically to the optical guide plate, and emitted from the optical guide plate. In connection with the conventional front light described above, the following proposal has been made. More specifically, the ratio of the reflective surface inclined by 45xc2x0 described above, which is the cause of loss of the light when the light is reflected by the liquid crystal panel or when the light passes again through the depth direction of the optical guide plate, or when ambient light passes through the optical guide plate, is set to at most ({fraction (1/20)}), so as to reduce the loss (Japanese Patent Laying-Open No. 11-202785).
Further, referring to FIG. 59, a proposal has been made in which prism-shaped grooves 112 are provided in optical guide plate 101, with one inclined surface of each groove is made to have the inclination in the range of 35xc2x0 to 55xc2x0 and the other inclined surface is made to have the inclination in the range of 60xc2x0 to 90xc2x0 (Japanese Patent Laying-Open No. 11-242222). The light beam proceeding in the optical guide plate encounters the inclined surface in the range of 35xc2x0 to 55xc2x0 as in the conventional examples, reflected therefrom and emitted to the outside of the optical guide plate. Further, referring to FIG. 60A, a method has been proposed in which the depth of the grooves is made gradually deeper further away from the light source so that the intensity of the emitted light becomes uniform, or referring to FIG. 60B, a method of adjustment has been proposed in which the pitch between the grooves is made gradually narrower further away from the light source, so that the intensity of emitted light is made uniform regardless of the positions.
In order to prevent degradation of display quality by moire fringes resulting from interference of the pitch of the prism structure and the pixel pitch of the display device, a proposal has been made in which the period of the prism structure is limited to the range of 1/(1.3+N) to (1/1.6+N) of the pixel period (Japanese Patent Laying-Open No. 11-260128).
Further, a proposal has been made in which an anti reflection film is provided on the first surface of the optical guide plate, so as to prevent lowering of display contrast caused by reflection at the first surface when the light is emitted from the optical guide plate (Japanese Patent Laying-Open No. 11-242220).
The optical guide plates of the conventional front lights each have the prism-shaped structure. Therefore, in most cases, the rib-shaped or fringe-shaped prism structure is visually recognized by the viewer, lowering the display quality. Further, as the light beam is reflected by the liquid crystal panel and again passes through the optical guide plate, the light beam is refracted to different directions at surfaces of different angles, resulting in the problem that double images are generated. Further, the above described prism optical guide plates all utilize the method in which the direction of progress of the light is changed by the regular reflection at the reflective surface of the prism, so that the light is emitted from the optical guide plate. Therefore, characteristic of the emitted light significantly vary along the direction of progress of the light beam in the optical guide plate. When the light beam is refracted at a corner of the optical guide plate and the refracted light enters the optical guide plate at an angle different from the remaining light beams, the light beam from the different angle appears as a strong emission line even after it is reflected by the prism, considerably degrading the display quality.
When a point light source such as an LED (Light Emitting Diode) is used as the light source, a proposal has been made in which the light is made uniform by a guide structure arranged at an end portion of the optical guide plate before entering the optical guide plate, as shown in FIGS. 61A and 61B. In the method in which the light is reflected at the prism-shaped projections at the second surface of the optical guide plate so as to extract the light out from the optical guide plate, it is necessary to make uniform the light spatially and, in addition, to make the pitch of reflective grooves of the light extracting mechanism of the edge optical guide plate (second optical guide plate) to be a prescribed value or smaller. Otherwise, significant unevenness results in the illumination. When the light entering the optical guide plate has wide angular distribution, the emitted light also comes to have wide angular distribution, and hence it becomes difficult to attain high brightness.
The front light of the type using the point light source shown in FIGS. 61A and 61B described above is suitable for reduction in size and weight. Accordingly, use of this type of front light has been increased, as the front light of a liquid crystal display device of personal digital assistants, represented by a mobile phone (Japanese Patent Laying-Open No. 10-260405).
The conventional front lights having the above described structures have the problems that the fringe-shaped prism structure of the optical guide plate is visually recognized, and display quality is degraded because of emission lines or unevenness. Further, it has been impossible to control angular distribution of the emitted light from the front light, and hence it has been difficult to ensure high brightness.
In the front light having such a structure as shown in FIGS. 61A and 61B using a point light source, it is the main object to obtain a display image of high brilliance with small power consumption. For this purpose, a structure has been used in which an edge optical guide plate (second guide plate) is arranged at a side edge of the main optical guide plate, and on a surface (fourth surface) of the edge optical guide plate that is opposite to the surface facing the main optical guide plate, reflective grooves for extracting light beams is provided. In this structure, referring to FIG. 62A, the light entering from an LED to the edge optical guide plate (second optical guide plate) will be a light beam K1 regularly reflected by the incline surface of the reflective groove 112, and a light beam K2 reflected and emitted to the outside and thereafter reflected by a reflector 155 and incident on the second optical guide plate from the outside. In order to suppress the loss of light at the reflection and diffraction, the angle formed by each of the two sides of the prism-shaped groove and a flat portion is made about 50xc2x0, as can be seen from FIGS. 62B and 62C.
When the above described light beams K1 and K2 proceed to the light emitting surface (third surface) of the second optical guide plate facing the main optical guide plate within a prescribed angular range, the beams are emitted from the light emitting surface and introduced to the main optical guide plate. When the light beams proceed to the light emitting surface out of the prescribed angular range, the light beams are regularly reflected at the light emitting surface, and proceed through the second optical guide plate. When the second optical guide plate is larger than a prescribed size, the ratio of the light beam regularly reflected at surfaces other than the light emitting surface of the second optical guide plate is high, and the ratio of light beams emitted to the outside is small. Therefore, when the second optical guide plate is larger than the prescribed size, efficiency, that is, the ratio of the light beam emitted from the light emitting surface and eventually introduced to the main optical guide plate to emitted from the point light source, is at a high level.
It should be noted, however, that the above described efficiency lowers when the two-dimensional size is small, as in the case of a front light of a liquid crystal display device mounted on a personal digital assistant, and hence it becomes difficult to obtain a display image of a desired brightness. In order to support the rapid development of the personal digital assistants, it has been strongly desired to enhance the efficiency described above, in the front light using the point light source.
An object of the present invention is to provide a small and compact, economically advantageous front light that does not generate emission lines or unevenness but provides display of high quality with high efficiency and uniform brightness, as well as to provide a reflective liquid crystal display device and a personal digital assistant using the front light.
The present invention provides a front light arranged in front of a display device, including: a main optical guide plate; a second optical guide plate having its longitudinal direction arranged to extend along widthwise direction of and at an end portion of the main optical guide plate, the second optical guide plate having an end surface facing the main optical guide plate and an opposite end surface, the opposite end surface including a plurality of prism-shaped grooves extending in depth direction, formed along the longitudinal direction; a point light source arranged at an end of the longitudinal direction of the second optical guide plate; and a reflective film coated on a surface of the prism-shaped grooves.
By this structure, the surface of the prism-shape grooves constituting the light extracting mechanism is coated with a reflective film. Therefore, at the second optical guide plate of which length is not longer than a prescribed length, reflective light with superior directivity can be obtained, with reduced loss of the light intensity. Here, as the light is scattered and reflected by the grooves, it is possible to make uniform the average light intensity averaged over all directions, at every position of the second optical guide plate.
As already described, when the two-dimensional size is small as in the case of a front light of a reflective liquid crystal display device mounted on a personal digital assistant, the above described efficiency becomes low without reflective film, and it becomes difficult to obtain the display image of a desired brightness. When the reflective film is provided on the surface of the prism-shape groove, no light beam goes out from the second optical guide plate through the surfaces of the groove, and the light is reflected by the reflective film, so that light beam of high directivity is emitted from the light emitting surface (third surface). The reflection loss generated at the time of reflection at the reflective film is as high as 10% or more. Therefore, if the second optical guide plate is longer than the prescribed length and the reflection is repeated, the efficiency becomes even lower than the second optical guide plate not having the reflective film. More specifically, when the second optical guide plate becomes longer than a prescribed value, the conventional second guide plate not having the reflective film on the surface of the grooves would have higher efficiency, as the propagation loss is smaller. The length that can assure improved efficiency when the reflective film is provided on the surface of the prism-shaped grooves is at most 70 mm to 100 mm, when the general reflection loss with the ordinary width size is about 10%, though it depends on the width of the second optical guide plate and on the magnitude of reflection loss at the reflective film. Therefore, when the length of the second optical guide plate is at most 70 mm to 100 mm, provision of the reflective film as a coating on the surface of the prism-shaped grooves is effective. The relation between the length of the second optical guide plate with the reflective film provided on the surface of the grooves and the length has not been known.
As the reflective film, generally, a metal film formed by vapor deposition or sputtering is used. It is possible to reduce the loss by placing a reflector plate or the like in place of the reflective film formed of a metal film. However, the light beam that once goes out from the grooves of the second optical guide plate, reflected by the reflector plate and returned to the groove portions come to have wide width in the direction of progress, and hence directivity is considerably degraded. It is essential to prevent the loss by preventing the light from going out from the second optical guide plate through the grooves, by applying the reflective film. Not only a film but any material may be used, provided that the material is flexible and capable of covering all the surfaces of the grooves, such as a reflective sheet, different from the rigid material such as the reflector plate. More specifically, the above described reflective film may include such a reflective sheet, a reflective film, a reflective tape, a coated film, a paste and the like.
By the above described structure, it becomes possible to provide uniform and highly bright display image of a mobile phone or the like, using a point light source of LED or the like. Generally, it is a common practice to provide a reflector from the side of the fourth surface of the second optical guide, covering the surface of the second optical guide plate to the main guide plate, so as to enhance the efficiency. A silver tape is provided on the inner surface of the reflector. When the reflective film described above is used, the efficiency can be improved, and therefore, the silver tape on the inner surface of the reflector may be eliminated. Further, the reflector itself may be eliminated.
In the front light according to the present invention described above, desirably, each of the two sides constituting the prism-shape forms an angle of 30xc2x0 to 38xc2x0 with a flat portion, when viewed two-dimensionally.
As the two sides constituting the prism-shape of the groove each form an angle of 30xc2x0 to 38xc2x0 with the flat portion, it is possible to obtain reflected light beam of high directivity, with small widthwise spread from the direction vertical to the second optical guide plate as the center. The angular range is suitable for the prism-shaped grooves with reflective film provided thereon, and such an angular range had not been considered for the conventional prism-shaped grooves without the reflective film (for such grooves, the angle has been about 50xc2x0). When the grooves are coated with the reflective film, it is possible to emit light with high directivity, with this small angle of 30xc2x0 to 38xc2x0, and hence brightness of the display image can be improved. Though it is easier to manufacture and less expensive to have the two sides of every prism-shaped groove at the same angle within the range of 30xc2x0 to 38xc2x0, it is not necessary that the two sides are at the same angle. Further, it is not necessary that all the grooves have the same angle.
In the front light of the present invention, preferably, an area coated with the reflective film of the second optical guide plate extends exceeding the end portion of the main optical guide plate when the main optical guide plate and the second optical guide plate are viewed in a direction from the second optical guide plate to the main guide plate.
Because of this structure, a portion where light is not introduced from the second optical guide plate is not generated in the main optical guide plate. Therefore, the light is emitted from the entire surface of the main optical guide plate to the display device, and unevenness of brightness and darkness as well as undesirable recognition of border lines between the bright and dark portions can be avoided.
In the front light of the present invention, the grooves may be provided such that the angle formed by the side constituting the prism-shape with the flat portion vary along with the distance from the point light source.
By this structure, when the intensity of light proceeding toward the light emitting surface varies with the distance from the point light source or when the angle toward the light emitting surface vary, it is possible to make uniform the light intensity, or to have the angles within a prescribed range near vertical to the light emitting surface, by varying the angle of the side of the prism-shape. When the angle is not changed, the prism has the shape of an isosceles triangle when viewed two-dimensionally. When the angle is to be changed as in the present structure, the shape may or may not be the isosceles triangle. Further, as the angle varies, the depth of the prism-shaped grooves also changes naturally.
In the front light of the present invention, the angle formed by a side constituting the prism-shape with the flat portion may be increased as the grooves are positioned further away from the point light source.
When the angle formed by a side of the groove and the flat portion is made constant regardless of positions in the second optical guide plate, a phenomenon is observed, in which the direction of emission of the light emitted from the light emitting surface (third surface) is inclined between an end portion near the point light source and a central portion away from the light source. The inclination of the direction of emission should be avoided, as it may cause uneven brightness. By changing the angle of inclination of the prism near the point light source and the central portion as described above, it becomes possible to limit the emission angle distribution to be within a prescribed range.
In the front light of the present invention, it is desired that pitch of the grooves is not larger than the pixel pitch of the display device.
In the main optical guide plate, the light propagating through the main optical guide plate is emitted, utilizing mirror surface reflection from the prism. Therefore, light beams proceeding in different directions in the main optical plate are emitted in different directions, when emitted from the main optical plate. At portions immediately corresponding to the bottom points of the grooves of the second optical guide plate, light beams reflected by the grooves and emitted approximately vertical from the end surface of the second optical guide plate are dominant. The light beam emitted approximately vertical from the end surface of the second optical guide plate are reflected by the inclined surfaces of the prism-shaped projections, which are approximately vertical to the direction of progress of the light beams and extending in the widthwise direction of the main optical guide plate, in the main optical guide plate, and emitted in approximately vertical direction, from the main optical guide plate. At portions between the bottom points of the grooves, light beams reflected by grooves on both sides and emitted obliquely from the end surface of the second optical guide plate are dominant. The light beams are directed obliquely to the prism-shaped projections of the main optical guide plate, and therefore, even after reflected at the inclined surfaces of the projected portions, the light beams are emitted not vertically but obliquely inclined manner with respect to the main optical guide plate. Even after reflected by the display device, the light beams that have been emitted vertically contribute more effectively to the front brightness, than the obliquely emitted light beams. Therefore, at central portions between the bottom points of grooves where light beams are not much emitted vertically, the front brightness becomes lower than the portions immediately corresponding to the bottom points, even when the emission intensity is the same. Therefore, fringe-shaped unevenness of brightness is observed. When the pitch between the grooves is made not larger than the pixel pitch, the unevenness of brightness can be made unrecognizable.
In the front light of the present invention, it is desired that the groove pitch is at most 0.3 mm.
By this structure, unevenness of brightness can be made comparable to or lower than the ordinary pixel pitch, and hence illumination recognized as uniform can be provided. Even when there is no information as to the pixels of the display device, the front light may be manufactured with this groove pitch, as the common pixel pitch is about 0.3 mm. Thus, a front light that can be used for various and many applications, including wide variety of display devices, can be provided. When very high uniformity of illumination is required, the groove pitch described above should desirably be 0.2 mm or smaller.
In the front light of the present invention, the end portion of the main optical guide plate on the side of the second optical guide plate is tapered from opposing sides such that the central portion of the end portion protrudes most to the second optical guide plate, the end surface facing the main optical guide plate of the second optical guide plate is tapered from opposing sides such that the central portion of the end surface is recessed most from the main optical guide plate, conforming to the tapered shape of main optical guide plate, and the main optical guide plate and the second optical guide plate are positioned with the tapered shapes fitted with each other.
By this structure, of the light beams entering from the point light source to the second optical guide plate, those reaching the tapered end surface of the second optical guide plate are regularly reflected, directed to the prism-shaped grooves coated with the reflected film and positioned on the opposite end surface, reflected therefrom and directed to the main optical guide plate. Those of the light beams that are not reflected at the tapered end surface of the second optical guide plate but refracted are partially reflected at a surface at the end portion of the taper of the main optical guide plate, which reflected light beams again enter the second optical guide plate, directed to and reflected by the surfaces of the prism-shaped grooves, and directed to the main optical guide plate. Thus, the light beams emitted from the point light source can be introduced with high efficiency to the prism-shaped reflective grooves coated with the reflective film, of the second optical guide plate. As a result, the light beams can be used with higher efficiency.
In the front light of the present invention, desirably, the reflective film is a thin film formed of any of aluminum, silver, gold, copper and platinum.
A reflective film having high reflectivity in the entire range of visible light and free of wavelength dependency is desired. A reflective film satisfying these conditions include aluminum (At) and silver (Ag) thin films. Platinum (Pt) may also be used, though it has slightly lower reflectivity. Further, emission spectrum of white LED generally used as the point light source has high peak on the blue side of shorter wavelength, as white color is attained generally by wavelength-converting blue LED by a fluorescent body. It is sometimes preferable to adjust white balance so as to lower reflectivity on the shorter wavelength side, so that emission spectrum becomes uniform regardless of the waveform. Gold (Au) and copper (Cu) may be used for this purpose, as it has reflectivity higher on the longer wavelength side and lower on the shorter wavelength side.
By using these metal films as the reflective film, it becomes possible to form a thin film having high reflectivity and high durability, easily by a known method such as vapor deposition or sputtering. To attain high reflectivity and high durability, the metal film may have the thickness of at least 25 nm and more desirably, at least 50 nm.
In the front light of the present invention, the reflective film may be formed on an underlying film formed on the surface of the prism-shaped grooves.
The main optical guide plate and the second optical guide plate are generally formed of resin materials. When a metal film is directly formed on the resin material, sometimes adhesion therebetween is not sufficient. By forming an underlying film such as SiO2 that provides sufficient adhesion both with the metal film and the resin material, and by coating the metal film thereon, sufficient adhesion and durability can be attained. Further, the interface between the underlying film and the metal film can be maintained in a flat, free-of-impurity state and hence increased reflectivity can be expected. When increase in reflectivity is desired, the index of refraction and the thickness of the underlying film may be controlled such that reflection at the interface between the resin material and the underlying film and reflection at the interface between the underlying film and the metal film interfere and reinforce with each other, thus providing a multi-layered reflection enhancing film.
In the front light of the present invention, the surface of the prism-shaped grooves may be roughened.
By the roughening process, adhesion of the reflective film can be improved. The roughening process may include blasting. The metal film may be coated with an underlying film interposed, on the roughened surface, or the metal film can directly be coated on the roughened surface, without the underlaying film.
In the front light of the present invention, a scattering member may be provided on the side of that surface of the second optical guide plate which opposes to the main optical guide plate.
By the scattering attained by the scattering member, light beams spatially made uniform can be introduced to the main optical guide plate.
The scattering member is effective to make uniform the light intensity spatially, when the pitch of the prism-shaped grooves is large.
In the front light of the present invention, desirably, an LED (Light Emitting Diode) is used as the point light source.
By the use of LED, the front light can be made compact, light, less power consuming, and driving voltage therefor can be lowered. As the driving voltage becomes lower, the number or scale of the battery to be mounted can be reduced. Thus, by the adoption of the LED, it becomes possible to use the front light in wider applications, for example, as a front light for personal digital assistant including mobile phones.
The present invention provides a reflective liquid crystal display device having a front light on a front side facing a viewer, the front light including a main optical guide plate; a second optical guide plate having its longitudinal direction arranged to extend along widthwise direction of and at an end portion of the main optical guide plate, the second optical guide plate having an end surface facing the main optical guide plate and an opposite end surface, the opposite end surface including a plurality of prism-shaped grooves extending in depth direction, formed along the longitudinal direction; a point light source arranged at an end of the longitudinal direction of the second optical guide plate; and a reflective film coated on a surface of the prism-shaped grooves.
In the reflective liquid crystal display device, as the front light described above is used, an image display of uniform brightness can be obtained with a compact and light power supply. Further, by changing the angle of the two sides forming the prism-shaped grooves of the second optical guide plate along with the distance from the point light source or by making shorter the pitch of the prism-shaped grooves, superior display quality, light weight, compact size and high efficiency can be attained.
In the reflective liquid crystal display device of the present invention, the two sides constituting the prism-shape may each form an angle of 30xc2x0 to 38xc2x0, with a flat portion, when viewed two-dimensionally.
By this structure, it becomes possible to improve directivity of the light beam proceeding from the second optical guide plate to the main guide plate, and hence brightness of the image displayed on the reflective liquid crystal display can be improved.
In the reflective liquid crystal display device of the present invention, desirably, an area coated with the reflective film of the second optical guide plate extends exceeding the end portion of the main optical guide plate, when the main optical guide plate and the second optical guide plate are viewed in a direction from the second optical guide plate to the main guide plate.
By this structure, unevenness in brightness is avoided in the image displayed on the liquid crystal display, and uniform brightness can be attained.
The present invention provides a personal digital assistant provided with a reflective liquid crystal display device including a front light, the front light including: a main optical guide plate; a second optical guide plate having its longitudinal direction arranged to extend along widthwise direction of and at an end portion of the main optical guide plate, the second optical guide plate having an end surface facing the main optical guide plate and an opposite end surface, the opposite end surface including a plurality of prism-shaped grooves extending in depth direction, formed along the longitudinal direction; a point light source arranged at an end of the longitudinal direction of the second optical guide plate; and a reflective film coated on a surface of the prism-shaped grooves.
By this structure, it is possible to make compact and light the personal digital assistant such as a mobile phone, or a PDF (Portable Document Format), and in addition, to provide display image having high uniformity and high brightness. In the personal digital assistant, it is desirable that a brightness adjustment volume is provided for adjusting brightness of the displayed image, by controlling power fed to the point light source. By the provision of the brightness adjustment volume dial, it becomes possible to make longer the interval between battery charge by saving power, for example, by reducing power fed to the point light source, that is, LED, where it is bright. Further, desired brightness of the display image can be attained anywhere, including a dark place.
In the personal digital assistant of the present invention, the two sides constituting the prism-shape may each form an angle of 30xc2x0 to 38xc2x0 with a flat portion, when viewed two-dimensionally.
By this structure, directivity of the light beam proceeding from the second optical guide plate to the main guide plate can be improved, and the brightness of the display image of the personal digital assistant can be improved.
In the portable digital assistant of the present invention, an area coated with the reflective film of the second optical guide plate extends exceeding the end portion of the main optical guide plate, when the main optical guide plate and the second optical guide plate are viewed in a direction from the second optical guide plate to the main guide plate.
By this structure, unevenness in brightness can be avoided on the displayed image of the personal digital assistant and uniform brightness can be attained.
In the personal digital assistant of the present invention, desirably, the length of the second optical guide plate is at most 100 mm.
This range of the length ensures that the second optical guide plate having the prism-shaped grooves coated with the reflective film attains higher efficiency than the second optical guide plate not having the reflective film coating. Further, this range of the length corresponds to a small size suitable for mounting on portable digital assistance. As a result, a display image with high brightness can be obtained. The length of the second optical guide plate is, more desirably, at most 70 mm and, for higher efficiency, it is desirable to be 50 mm at most.
In the portable digital assistant of the present invention, the width of the second optical guide plate should desirably be at most 10 mm.
By this structure, the display portion effective for displaying information can be enlarged, providing easily viewable image display. The width of the second optical guide plate should desirably be in the range of 2 mm to 7 mm. When the width of the second optical guide plate is smaller than 2 mm, it is difficult to supply light beams of uniform brightness over the widthwise direction of the main optical guide plate. When the width exceeds 7 mm, the area of the display portion for displaying image would be limited. In view of the foregoing, the width of the second optical guide plate is, more desirably, in the range of 3 mm to 6 mm.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.